Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then collects residual toner not transferred and remaining on the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
Such fixing device may include a fixing roller heated by a heater, and a pressing roller pressed against the fixing roller to form a nip therebetween. As a recording medium bearing a toner image passes through the nip, the fixing roller and the pressing roller apply heat and pressure to the recording medium to melt and fix the toner image on the recording medium. Thereafter, the recording medium bearing the fixed toner image is discharged from the nip.
With a configuration in which an induction heater is used as the heater that heats the fixing roller, the fixing roller is constructed of a hollow cylindrical fixing sleeve including a heat generating layer that generates heat by a magnetic flux from the induction heater and a support roll inserted into the fixing sleeve. The support roll includes a foam layer made of silicone rubber foam and constituting the outer circumferential surface of the support roll. As a method of securing the support roll to the fixing sleeve, it is known to apply an adhesive to the entire outer circumferential surface of the support roll that adheres the outer circumferential surface of the support roll to the inner circumferential surface of the fixing sleeve.
However, such configuration of the fixing roller has a drawback in that as the induction heater heats the fixing roller, air trapped between the outer circumferential surface of the support roll and the inner circumferential surface of the fixing sleeve thermally expands and cannot be exhausted from the fixing roller, increasing repulsive load, that is, pressure generated by the pressing roller pressing against the fixing roller, applied at the center of the fixing roller in the axial direction thereof. As a result, the fixing roller with the increased repulsive load may crease the recording medium that contacts the fixing roller as the recording medium passes through the nip formed between the fixing roller and the pressing roller.
Referring to FIG. 1, a detailed description is now given of a mechanism that causes such creasing of the recording medium.
FIG. 1 includes diagram A1 showing arrows F1 that indicate a force applied to the recording medium and arrows F2 that indicate a conveyance speed at which the recording medium is conveyed through the nip at an ambient temperature; graph B1 showing a pressure distribution in the axial direction of the fixing roller of pressure applied by the fixing roller to the recording medium at the nip at the ambient temperature; diagram A2 showing arrows F3 that indicate a force applied to the recording medium and arrows F4 that indicate a conveyance speed at which the recording medium is conveyed through the nip at a fixing temperature at which a toner image is fixed on the recording medium; and graph B2 showing a pressure distribution in the axial direction of the fixing roller of pressure applied by the fixing roller to the recording medium at the nip at the fixing temperature.
Generally, at the ambient temperature as shown in diagram A1, with the pressing roller having a hand drum shape, the lateral ends of the recording medium in the axial direction of the fixing roller are conveyed at a speed higher than a speed at which the center of the recording medium in the axial direction of the fixing roller is conveyed, as shown by the arrows F2 in which the longer arrows show the higher speed and the shorter arrows show the slower speed. Accordingly, a substantially identical pressure is applied by the fixing roller to the recording medium over the axial direction of the fixing roller as shown in diagram B1 and the fixing roller applies the force in the two opposite directions as shown by the arrows F1 to the recording medium, thus stretching the recording medium outward in the axial direction of the fixing roller and therefore preventing creasing of the recording medium.
However, as the induction heater heats the fixing roller, air trapped between the fixing sleeve and the support roll thermally expands, increasing the repulsive load applied to the center of the fixing roller in the axial direction thereof as shown in graph B2. As a result, the recording medium is conveyed at a higher speed at the center of the fixing roller in the axial direction thereof as indicated by the arrows F4 in diagram A2, thus offsetting the force that stretches the recording medium outward in the axial direction of the fixing roller as indicated by the arrows F3 in diagram A2 and therefore creasing the recording medium.